A β-sialon phosphor activated with a rare earth element has been reported in Kokai (Japanese Unexamined Patent Publication) No. 60-206889. This β-sialon phosphor emits a green fluorescence at 525 to 545 nm by the excitation of ultraviolet light at 315 nm or less. Recently, studies are being aggressively made on a white light emitting diode using a near-ultra-violet-to-blue light emitting diode as the excitation source, and a phosphor capable of strongly emitting light with this excitation wavelength is demanded. The green light emitting β-sialon phosphor also requires a green fluorescence strongly emitting with the above-described excitation wavelength range. However, the phosphor described in Kokai No. 60-206889 is difficult to be applied, because the optimal excitation wavelength is present in the ultraviolet region. On the other hand, it has been reported in Kokai No. 2005-255895 that a β-sialon phosphor becomes a material capable of emitting a strong green fluorescence by the excitation of near ultraviolet-to-blue light. This phosphor is usable for a fluorescent display tube (VFD), a field emission display (FED), a plasma display panel (PDP), a cathode ray tube (CRT), a white light emitting diode (LED) and the like and is expected as a phosphor causing little reduction of brightness. Incidentally, in Kokai No. 2005-255895, a substantially acicular β-sialon phosphor is only disclosed.